Tethered buoyant support for risers to a floating production vessel

ABSTRACT

A mid-water tethered buoyant support assembly for a riser system for use in water is described to bring fluids from seabed equipment to a production vessel at the surface. The tethered buoyant support assembly comprises at least two tethers ( 6 ) from seabed anchors, at least one beam assembly ( 2 ) extending between and connected to the tops of the tethers, buoyancy means ( 7 ) to maintain tension in the tethers, and hangers ( 10 ) for lower riser portions mounted at spaced positions along the beam assembly, each hanger ( 10 ) being positioned so that the line of action of the tension due to the weight of the suspended lower riser portion is close to or on a line extending between the connections of the beam to the tethers ( 6 ) to minimize or eliminate turning moment to the beam assembly ( 2 ) tending to cause rotation of the beam around its major axis as a result of the weight of the suspended lower riser portion. The assembly is particularly designed for use in deep water.

This invention relates, to a tethered buoyant support for risers to afloating production vessel, the tethered buoyant support being at amid-water location for supporting the riser pipe catenaries.

A lower J-shaped catenary extends from the seabed to the support, and anupper U-shaped catenary extends from the support to the vessel floatingat the surface. The riser system with a single buoyant support cancomprise multiple riser pipes, all of them with lower and uppercatenaries. Previous similar catenary riser systems have been describedin EP 251488 and UK 2295408.

In all water depths, the upper catenary is usually fabricated fromflexible pipe or ‘flexpipe’. Flexpipe is able to absorb vessel motion inwaves without being vulnerable to fatigue failure, and has been used formost risers to floating production vessels in service in 1998. Flexpipeis here defined as high pressure flexible pipe, which usually includeshelical high-strength windings (such as steel or possibly carbon fibre)to re-inforce polymer tubes or an elastomer matrix.

In deep water (greater than 500 m) it is desirable to fabricate thelower catenary from steel pipe rather than flexpipe, due to the steelpipe having long length relative to its diameter (the length beingaround 1000 times greater than the diameter, or more). Steel catenaryriser (SCR) technology to a tension leg platform (TLP) is described in atechnical paper entitled ‘Design and Installation of Auger SteelCatenary Risers’ presented at the Offshore Technology Conference inHouston, May 1994, paper number OTC 7620. UK 2295408 describes theapplication of SCRs with a tethered buoyant mid-water support, ratherthan to a TLP.

Installation of tethered buoyant supports in 130 m water depth offshoreNorth West Australia is described in ‘Installation of the Griffin FPSOand Associated Subsea Construction’, paper presented at the FloatingProduction Systems Conference, in London, Dec. 8-9, 1994. Eachcylindrical buoy was 3.7 m diameter and up to 14 m long with chaintethers from each end down to a seabed base. The buoy was positionedapproximately 45 m below the sea surface. The buoys carried arches forsupporting flexpipe risers and umbilicals, and the arch radius wasapproximately 3 m, with the buoy cylinder positioned centrally under thearches (at least before installing flexpipe risers).

In deep water, the tension at the top of the lower J-shaped catenaryextending from the mid-water support to the seabed can be very large dueto the submerged weight of the long length of the lower catenary pipe.The paper OSEA-94113, ‘A Hybrid Riser for Deep Water’ presented at theOffshore South East Asia Conference, Singapore, Dec. 6-9, 1994, suggeststhat multiple SCRs from a mid-water support located 100 to 150 m belowsurface in 1200 m depth, will have a combined submerged weight of 1200tonnes. The paper OTC 8441—‘Integrated Asymmetric Mooring and HybridRiser System for Turret Moored Vessels in Deep Water’, presented at theOffshore Technology Conference, Houston, May 5-8, 1997—describes atethered riser buoy in 1000 m water depth for supporting up toapproximately 800 tonnes of load from 15 risers and umbilicals. PaperOTC 8441 suggests that a concrete buoy for this application should be 8m diameter and 80 m long, and should generate 1200 tonnes of tethertension to provide adequate lateral stability.

The problem with hanging a load of 800 to 1200 tonnes from a circularsection buoy with a centrally-positioned support arch of 3 to 4 m radiusis that the moment of up to 4800 tonne-meters will tend to rotate thebuoy. Also, the rotation could bend the upper ends of the risers unlessthey are hanging from a ‘hinged’ (i.e. free) support.

Even if the lower riser portion submerged weight can be reduced byadding a low density coating, or by using pipe-in-pipe construction witha gas-filled annulus, the hanging weight is still likely to be hundredsof tonnes.

The invention has therefore been made with these points in mind.

According to the invention there is provided a mid-water tetheredbuoyant support assembly for a riser system for use in water to bringfluids from seabed equipment to a production vessel at the surface, thetethered buoyant support assembly comprising at least two tethers fromseabed anchors, at least one beam assembly extending between andconnected to the tops of the tethers, buoyancy means to maintain tensionin the tethers, and hangers for lower riser portions mounted at spacedpositions along the beam assembly, each hanger being positioned so thatthe line of action of the tension due to the weight of the suspendedlower riser portion is close to or on a line extending between theconnections of the beam to the tethers, to minimise or eliminate turningmoment to the beam assembly tending to cause rotation of the beam aroundits major axis as a result of the weight of the suspended lower riserportion.

Such an assembly supports the lower riser weight with minimum tendencyto cause rotation of the tethered buoyant support. In addition it ispossible to provide a large amount of adjustable buoyancy at the supportform which is readily fabricated. Further, there is resistance torotation of the support when flexpipe upper catenaries are added.

Advantageously, the distance between the line of action of the tensionof a lower riser portion and the line extending between the tops of thetethers is at most one quarter of the distance from the centre ofbuoyancy of the buoyancy means to the tops of the tethers. Moreadvantageously, the distance between the line of action of the tensionof any lower riser portion and the line extending between the tops ofthe tethers is at most one twentieth of the distance from the centre ofbuoyancy of the buoyancy means to the tops of the tethers.

The tethered buoyant support may include joining and/or guiding and/oraligning means for upper riser portions mounted on the beam structure atspaced positions corresponding to the hangers.

The vertical tethers can be similar to the tubular tethers used forTLPs, which are generally steel tubes and have elastomeric bearings atthe connection to the seabed anchors. Similarly, the connections of thetethers to the beam can be elastomeric bearings.

The horizontal beam structure can be two tubes around 2 m diameter andspaced around 4 m apart by minor tubular members in the manner of abraced truss around 50 m in length, and the hangers can be similar tothose described in European patent EP 0,251,488 or UK patent application2,323,876. The means for joining or guiding or aligning the upper riserportions to their corresponding lower riser portions can comprise archesfor supporting flexible pipe, or inverted U-shaped piping spools, orfunnels or guide posts for aligning connectors.

The main buoyancy tanks can be circular cylinder-shaped with the majoraxis vertical or rectangular block-shaped, and with the attachment tothe beam at the centre of the lower face. The tanks may have dimensionsaround 20 m high×10 m diameter (1570 cu.m. displacement) if this largeamount of buoyancy is needed, depending on the total riser weight to besupported. The inside of the tanks can be partitioned to allowprogressive increase of the buoyancy by de-ballasting pairs ofpartitions to maintain the buoy and beam close to vertical. Eachde-ballastable compartment has suitable valves to allow injection of airor nitrogen to the top, and ejection of contained water at the bottom,with minimum overpressure of the gas above external water pressure.

Specific embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is an isometric view of an entire floating production systemshowing multiple riser pipes to/from the seabed.

FIG. 2 is an isometric view of the beam structure with tethers andbuoyancy tanks at each end.

FIG. 3 is an end view of the beam showing the relative position of thetether bearings, buoyancy, and the applied riser loads.

Referring to FIG. 1, the production vessel 1 is floating on the seasurface. A mid-water support in the form of a beam structure 2 hassupport arches 3 for flexpipe upper riser portions 4. Lower riserportions 5 extend down to the seabed. Tethers 6 maintain the beamstructure at the desired depth and buoyancy tanks 7 support the weightof the entire assembly including the riser tensions and keep the tetherstaut. Guy lines 8 help to balance the lateral component of lower risertension and prevent lateral movement due to water current.

FIG. 2 is an isometric view of a beam structure 2 attached to tethers 6by elastomeric bearings 9. The beam 2 supports arches 3 and hangers 10for single line risers, and three arches 3 associated with hanger 11 fora riser bundle containing three lines. Another possible reason for asingle lower riser portion having multiple associated arches is that thelower riser portion is large, say 24″, and the upper flexpipe riserportions having limited diameter, say 16″ maximum. Hangers 10 and 11 mayhave hinged or elastomeric bearing attachment to the beam structure topermit hanger alignment with the lower riser portions (only centre-linepositions 12 of the lower risers are shown). The centre-line positions12 are equivalent to the lines of action of lower riser tensions at thehangers 10 and 11. Buoyancy tanks 13 are mounted on arms 14 integralwith the beam 2, and are positioned above the tethers 6. Partitions 15in the buoyancy tanks 13 provide some stiffening, some redundancy if onebuoy compartment fails and floods, and may allow finer adjustment ofbuoyancy by de-ballasting segments only. Guy lines 8 have means 16 foradjustment of their tension where they attach to the beam 2.

FIG. 3 shows the beam 2 connected to tethers 6 by bearings 9. Label ‘B’represents the top of the tether, and the second tether will have acorresponding point ‘B’. When a lower riser is installed, the line ofaction of its tension ‘T’ (centre-line 12) exerts a moment of ‘T timesa’ trying to rotate the beam. Distance ‘a’ is between the line of actionof the tension, and the line extending between the tops of the tethers(of which point ‘B’ is an end view) and is preferably less than 1.5 m,and more preferably less than 0.8 m. This tendency for the beam 2 torotate will try to move the centre of buoyancy (located at distance ‘L’above point ‘B’) of the buoyancy tanks 13 away from their normalposition vertically above point ‘B’. The buoyancy force will then startto generate an opposing moment, and will reach a stable position wherethe returning moment due to the displaced centre of buoyancy balancesthe moment arising from the lower riser tension ‘T times a’. If ‘a’ issmall and ‘L’ is large, then there will be very little rotationalmovement of the beam 2. Preferably, L is at least 3 m, and morepreferably at least 5 m. For example, L could exceed 10 m if the tanks13 are 20 m high as described above.

When a flexpipe upper section 4 is added over arch 3 to connect thelower riser portion to the surface vessel, its catenary will exert atension ‘t’ which is less than lower portion tension ‘T’. It will act atmoment arm ‘b’ from point ‘B’, and will act to counter some of themoment ‘T times a’, thus bringing the centers of buoyancy of thebuoyancy tanks 13 back closer to their starting position, verticallyabove points ‘B’. Thus, as seen in FIG. 3, the sum of the moments causedby the tensions of the upper and lower riser portions about the lineextending between the points ‘B’ is minimized or eliminated. That is:

T·a−t·b≈0.

The lower risers portions 5 can be from flexpipe or steel, and the anglebetween a lower riser portion centre-line 12 (representing the line ofaction of its tension at its approach to its support 11) and vertical islikely to vary as listed below:

Angle of centre-line Type of lower riser portion 12 to verticalFlexpipe/umbilical <5 degrees Steel pipe (4″ to 8″ NB) around 10 degreesSteel pipe (>10″ NB) >15 degrees

If the lower riser portions 5 for a particular project have similarangles of centre-line 12 to vertical at the approach to their hangers11, may be possible to reduce the turning moments ‘T times a’ and ‘ttimes b’ to lower values, as described below.

FIG. 2 shows the beam 2 offset, or ‘cranked’, in the horizontal plane,so that the hangers can be closer to the line extending between the topsof the tethers ‘B’. It may be advantageous to also offset the beam 2 inthe vertical plane. The lines of action of the tensions ‘t’ and ‘T’ inthe upper and lower riser portions are shown in FIG. 3. If thesecentre-lines are extended backwards, they intersect at a point 20 abovethe beam 2 and support arch 3. The turning moments ‘T times a’ and ‘ttimes b’ will be reduced to lower values if the beam 2 is offsetdownwards by around 5 meters. This will bring the intersection pointbetween the lines of action of the tensions ‘t’ and ‘T’ closer to theline extending between the tops of the tethers ‘B’, thus reducing anytendency to rotate the beam 2.

The amount of horizontal and vertical plane offset, or ‘crank’, in thebeam 2 for a particular water depth/riser size/etc. must be determinedduring detail design following evaluation of:

a) the forces acting at the mid-water tethered buoyant support,

b) the stresses developed in the beam, and

c) the cost-effectiveness of introducing greater complexity to beamfabrication.

FIG. 4 of European patent no. EP 251488 shows some risers passing backunder the beam structure rather than laying away from it, as shown inthe present FIG. 1. Beam structure 2 can support a riser which passesunder it (not shown here), and which has a short length of flowlinelying on the seabed to equipment under the floating vessel 1. In thatcase the centre-line 12 in FIG. 3 would still be spaced at smalldistance ‘a’ on the right-hand side of point ‘B’, but would cross thecentre-line of tether 6 at a relatively short distance below point ‘B’.Beam structure 2 would still be cranked in the direction shown in FIG.2, as the riser hang-off operation would approach the hanger 10 from thesame side. A detailed description of this operation where the riserpasses under the beam 2 was given in Offshore Engineer magazine, July1987, page 41.

Another variation for riser hang-off would be where long flowlinesand/or long export lines approach the beam structure from oppositesides. In this case, where the hang-off operations are on opposite sidesof the beam, the corresponding hangers 10 should also be on oppositesides of the beam 2. In this case, a single riser support system wouldsupport lines approaching from both sides rather than having two risersupport systems as shown in FIG. 1. The beam 2 would also need to becranked in both directions; preferably symmetrically with, say, anexport line at each end (from one direction) and all the flowlines inthe centre section (from the opposite direction). However, all theflexpipe links 4 would still leave the beam in the same direction. Forthose positions where the flexpipe link and the hanger for lowerJ-catenary are on the same side of the beam, the arch 3 and its supportwill need to be added after the lower J-catenary has been hung off.

In another embodiment of the invention, the main part of the buoyancywhich maintains tension in the tethers can be located at, near or aroundthe top ends of the tethers themselves, rather than above the tethers.This has the advantage of increasing the clearance between theproduction vessel mooring lines and the tethered buoyant riser supportassembly but has the disadvantage that the buoyancy will not oppose anyturning moment. In this case the beam has fixed connections at or nearthe tops of the tethers plus buoyancy means. It may be possible to makethe tethers and any guy lines from relatively low cost, synthetic fibreropes. It remains necessary to prevent application of a large turningmoment to the beam (tending to cause rotation of the beam around itsmajor axis) when the high load of the lower riser portions is applied tothe hangers.

When laying an offshore pipeline towards a seabed target area which maybe only 3 meters long by 3 meters wide, the lay-vessel must know itsposition with respect to where to cut the pipeline (which is fabricatedfrom 12 meter or 24 meter lengths). The cut must be made, and the‘lay-down head’ welded to the end, so that when the end of the pipelinehas travelled over the curved ramp or ‘stinger’, the end of the line islaid down in the target area. Gauging of the ‘distance-to-target’ can bedone using sonar methods, but there is a working tolerance ofapproximately+/−1 meter.

When laying towards a submerged tethered riser support into hangers 10,the effective width of the hanger target can be increased by addingangled guide arms which act to ‘funnel’ the riser into the requiredposition. These guide arms can be detachable, and can be installed at aselected hanger position by a diver or an ROV.

The ‘distance-to-target’ can only be gauged within a tolerance ofapproximately +/−1 meter, and the J-catenary geometry of the lower riserportion 5 will in some cases be able to accept this variation in lengthwithout causing excessive bending stress in the ‘sag-bend’. If the lowerriser portion length must be precisely controlled to keep bending stresswithin a certain limit (i.e. the catenary geometry can not absorb thepotential length variation), then it may be necessary to provide hangers10 and 11 with adjustment means to accommodate the variation ofJ-catenary effective length.

Hangers 10 and 11 can be attached to beam structure 2 by linearadjustment means (not shown) which can vary the position of the hangeralong the line of action 12 by approximately plus/minus 2 meters afterlower riser portion 5 hang-off. The linear adjustment means can besupported temporarily by a hydraulic actuator, which can change theelevation of the hanger 10 and 11 with respect to the beam 2. Afteradjusting the height of the hanger, the adjustment means can be lockedin position by adding pins in the nearest ‘match’ of a series of holes.Alternatively, the adjustment means can follow the principle of atypical ‘screw jack’, rather than a ‘pin-lockable-slide’ in conjunctionwith a temporary hydraulic jacking actuator.

Another method of providing adjustment would be to set the hanger 10 ata relatively low position, install the lower riser portion 5 and liftits upper end using the lay vessel winch until the weight-support-flangeat the end of the line is at the correct position. A support collar ofhalf-shells, made up to the required length, could then be added to takeup the distance between the weight-support-flange and the hanger.

A further alternative, to ensure that the riser portion 5 of aparticular flowline or pipeline is cut to the correct length, is tolower the top end of the riser pipe catenary with at least 3 m of extralength attached, down to the hanger position. This lowering activitywould be done, for either a seabed lay-down or a mid-water hang-off, byusing a winch line from the pipelay vessel. Previous analysis will havepredicted a desired top tension, top angle to vertical, and touch-downpoint at the seabed for this particular steel catenary riser. The winchline holding the riser weight can be adjusted to give the requiredtension, or angle, or touch-down point, and an ROV or diver can mark thenecessary cut position relative to the hanger 10,11. After retrievingthe riser top back to surface, the catenary portion 5 should be cut tothe required length for attachment of the hanger flange and lower partof a connector to ease future connection to the corresponding flexpipeupper portion 4 of the riser. Before lowering the top end of the riserportion 5 back down to its hanger 10 or 11, consideration must be madeof any hydrotesting that may be required for a complete flowline andriser. This testing may need a pig trap to be installed at the top ofthe catenary portion 5 to allow controlled flooding, prior to testing orattaching the flexpipe portion 4.

There have been two types of buoyant mid-water supports for flexpipecatenary risers to date. The first type is used for ‘steep’ riserconfigurations where the lower riser portion is attached at its lowerend to a fixed riser base on the seabed, and the mid-water support withriser arch is ‘tethered’ in position by the flexpipe itself. This typeof riser is usually installed in one piece with the mid-water supportattached, and lowered simultaneously with the riser. The second type isused for supporting ‘lazy’ riser configurations where the lower catenarytouches down tangentially at the seabed. This type can also be installedsimultaneously with the riser pipe, but when used to support a largenumber of risers, it is more usual to pre-install the mid-water supportwith arches. The pre-installation activity for six mid-water supports isdescribed in the previously noted reference at the top of page 2,related to the Griffin field facilities off Australia. The improvementsdescribed in this application relate only to pre-installed tetheredbuoyant riser supports which have a tether system attached to seabedpoints of fixity, and to which the risers are installed inclose-to-catenary configuration with tangential touch-down at seabedafter mid-water buoy installation is complete.

At some time after the tethered buoyant support has been installed, atether may be damaged and may need to be replaced. This replacementoperation can be made easier if additional fixing points for the ends ofa replacement tether are already provided at both the seabed anchors andat the ends of beam 2. After installing a new tether, the old damagedone can be safely removed. There is a philosophy for tethered (usuallymanned) platforms to be installed with at least two tethers pernecessary anchor point, so that if one tether fails, the other preventscatastrophic instability and failure of the platform. In the case of atethered buoyant riser support, each tether is likely to be very strongand damage is likely to cause only partial loss of strength. This damagewould probably be detected during periodic ROV inspection, and anassessment can be made of the urgency for its replacement. The veryunlikely failure of a riser support system may lead to failure of alower catenary riser pipe 5, but major release of hydrocarbons to thesea would be prevented by numerous near-wellhead valves located bothabove and below the seabed.

In FIG. 3, the arch 3 has one end close to tangential with thecentre-line 12 to allow alignment for near-vertical connection of anupper flexpipe portion 4 to its corresponding lower catenary portion 5.It should be noted that previous arches over tethered buoyant risersupports (such as those described for the Griffin field facilities inthe reference at the top of page 2) were located close-to-centrally withrespect to the near-vertical line of the tethers. That is, the centre ofthe radius of each arch is close to the plane of the two tethers. In theend view of the beam shown in FIG. 3, the arch 3 is significantly offsetwith respect to the centreline of the tether 6. This allows thecentre-line 12 to be close to (or on) a line extending between theconnections 9 of the beam to the tethers, thus greatly reducing thetendency for the beam to rotate when a lower catenary portion 5 is hungoff at its corresponding hanger 10,11.

In the book ‘Floating Structures: a guide for design and analysis’prepared by the (UK) Centre for Marine and Petroleum Technology in 1998and published by Oilfield Publications Limited, Chapter 13 is entitled‘Flexible Risers and Umbilicals’. This chapter includes a descriptionand drawing (FIG. 13.11) of a typical mid-water support. The drawingshows the attachment point of the tether at the far side of the archcentreline from the riser leg that descends to the RBM (Riser BaseManifold) on the seabed. In this position, any high load developed bythe hanging weight of the lower riser catenaries down to the seabed willgenerate a greater turning moment than if the tether had been located ata central position. The present invention recommends positioning theline of action of the hanging weight of the lower catenaries close tothe plane containing the (extended) centrelines of the main tethers inorder to minimise the associated turning moment.

FIGS. 2 and 3 herein show the main buoyancy tanks 13 positioned abovethe tethers 6. It may be advantageous to locate trim buoyancy tanks (notshown) along the upper tubular member of beam 2 and under the arches 3.These trim tanks could be used for fine adjustment during or afterinstalling upper riser portions 4. In FIG. 3, the tension ‘t’ from upperriser portion 4 is tending to rotate the beam 2 in an anti-clockwisedirection relative to the tether attachment point ‘B’, and this tendencycan be counteracted by adjustment of trim tank buoyancy positioned underthe arch 3. The effectiveness of any trim tank buoyancy is obviouslygreater if the centre of buoyancy is located further to the left oftether attachment point ‘B’.

What is claimed is:
 1. A mid-water tethered buoyant support assembly fora riser system for use in water to bring fluids from seabed equipment toa production vessel at the surface, the tethered buoyant supportassembly comprising: at least two tethers extending from seabed anchors,wherein said tethers are located in a single plane; a beam assemblyextending between and connected to the tops of said tethers atconnections, such that the connections of said tethers to said beamassembly are disposed along said beam assembly to define a lineextending between the connections; buoyancy means attached to said beamassembly for buoyantly supporting said beam assembly, so as to maintaintension in said tethers; and hangers for suspending lower riserportions, said hangers being mounted at spaced positions along said beamassembly and positioned so that lines of action of tension due to theweight of the suspended lower riser portions are closely adjacent to, oron, the line extending between the connections of said tethers to saidbeam assembly, to minimize or eliminate a turning moment imparted to thebeam assembly, which tends to cause rotation of the beam assembly aroundits major axis, as a result of the weight of the suspended lower riserportions.
 2. An assembly as recited in claim 1, wherein the line ofaction of the tension due to the weight of the suspended lower riserportions is no more than 1.5 m from the line extending between theconnections of said tethers to said beam assembly.
 3. An assembly asrecited in claim 1, further comprising arches, over which upper flexibleportions of the riser system are laid, and the upper flexible portionsare joined to the lower riser portions at one end of said arches, themajor axes of the center of radius of said arches being parallel to, butoffset from, the line extending between the connections of said tethersto said beam assembly.
 4. An assembly as recited in claim 3, whereinsaid beam assembly comprises a pair of tubular members, one of whichsupports said hangers, and the other of which is displaced therefrom andsupports said arches, so as to minimize or eliminate a turning moment onsaid beam assembly as a result of the weight of the suspended lowerriser portions.
 5. An assembly as recited in claim 1, wherein a centerof buoyancy of said buoyancy means is above the line extending betweenthe connections of said tethers to said beam assembly.
 6. An assembly asrecited in claim 5, wherein a distance of the center of buoyancy of saidbuoyancy means is at least three meters above the line extending betweenthe connections of said tethers to said beam assembly.
 7. An assembly asrecited in claim 6, wherein the distance between the lines of action ofthe tension of the lower riser portions and the line extending betweenthe connections of said tethers to said beam assembly, is at most onequarter of the distance from the center of buoyancy of said buoyancymeans to the line extending between the connections of said tethers tosaid beam assembly.
 8. An assembly as recited in claim 6, wherein thedistance between the lines of action of the tension of the lower riserportions and the line extending between the connections of said tethersto said beam assembly, is at most one twentieth of the distance from thecenter of buoyancy of said buoyancy means to the line extending betweenthe connections of said tethers to said beam assembly.
 9. An assembly asrecited in claim 5, wherein said at least two tethers comprise a pair oftethers, one at each end of said beam assembly, and said buoyancy meanscomprises a pair of buoyancy tanks, each positioned above a respectiveone of said pair of tethers.
 10. An assembly as recited in claim 5,wherein the center of buoyancy of said buoyancy means is at least fivemeters above the line extending between the connections of said tethersto said beam assembly.
 11. An assembly as recited in claim 1, whereinthe assembly is capable of being used in water having a depth greaterthan five hundred meters.
 12. An assembly as recited in claim 1, whereinthe line of action of the tension due to the weight of the suspendedlower riser portions is no more than 0.8 meters from the line extendingbetween the connections of said tethers to said beam assembly.
 13. Amid-water tethered buoyant support assembly for a riser system for usein water to bring fluids from seabed equipment to a production vessel atthe surface, the tethered buoyant support assembly comprising: at leasttwo tethers extending from seabed anchors, said tethers being located inand defining a single plane over at least a portion of their length; abeam assembly extending between and connected to the tops of saidtethers; buoyancy means attached to said beam assembly for buoyantlysupporting said beam assembly, so as to maintain tension in saidtethers; and hangers for suspending lower riser portions, said hangersbeing mounted at spaced positions along said beam assembly, said hangersbeing positioned closely adjacent to, or on, the plane defined by saidtethers in order to minimize or eliminate a turning moment applied tosaid beam assembly, which tends to cause rotation of said beam assemblyaround its major axis, as a result of the weight of the suspended lowerriser portions.
 14. An assembly as recited in claim 13, wherein adistance between each of the hangers and the plane defined by saidtethers is at most one meter.
 15. A mid-water tethered buoyant supportassembly for a riser system for use in water to bring fluids from seabedequipment to a production vessel at the surface, the tethered buoyantsupport assembly comprising: at least two tethers extending from seabedanchors; a beam assembly extending between and connected to the tops ofsaid tethers; buoyant supports attached to said beam assembly forbuoyantly supporting said beam assembly, so as to maintain tension insaid tethers; hangers for suspending lower riser portions, said hangersbeing mounted at spaced positions along said beam assembly; and upperriser supports for suspending upper portions of the riser system,wherein said beam assembly comprises a pair of elongate members, one ofwhich supports said hangers, and the other of which is displacedtherefrom and supports said upper riser supports, so as to minimize oreliminate a turning moment on said beam assembly as a result of theweight of the suspended lower riser portions.
 16. An assembly as recitedin claim 15, wherein the upper riser portions of the riser system areflexible.
 17. An assembly as recited in claim 15, wherein the upperriser supports comprise at least one of arches, inverted U-shaped pipingspools, funnels, and guide posts.
 18. An assembly as recited in claim15, wherein said tethers are located in a single plane.
 19. A mid-watertethered buoyant support assembly for a riser system for use in water tobring fluids from seabed equipment to a production vessel at thesurface, the tethered buoyant support assembly comprising: at least twotethers extending from seabed anchors; a beam assembly extending betweenand connected to the tops of said tethers; hangers attached to said beamassembly for suspending lower riser portions; and upper riser supportsattached to said beam assembly for suspending upper portions of theriser system, wherein said hangers and said upper riser supports arepositioned in a radial direction relative to said beam assembly suchthat the following condition is met: T·a−t·b≈0 where T equals thetension due to the lower riser portions, a equals the radial distancefrom the line of action of T to the line extending between theconnections of said tethers to said beam assembly, t equals the tensiondue to the upper portions of the riser system, and b equals the radialdistance from the line of action of t to the line extending between theconnections of said tethers to said beam assembly.
 20. An assembly asrecited in claim 19, wherein the upper riser portions of the risersystem are flexible.
 21. An assembly as recited in claim 19, wherein theupper riser supports comprise at least one of arches, inverted U-shapedpiping spools, funnels, and guide posts.
 22. An assembly as recited inclaim 19, wherein said tethers are located in a single plane.
 23. Amid-water tethered buoyant support assembly for a riser system for usein water to bring fluids from seabed equipment to a production vessel atthe surface, the tethered buoyant support assembly comprising: at leasttwo tethers extending from seabed anchors, said tethers being located inand defining a single plane over at least a portion of their length; abeam assembly extending between and connected to the tops of saidtethers; buoyancy means attached to said beam assembly for buoyantlysupporting said beam assembly, so as to maintain tension in saidtethers; and hangers for suspending lower riser portions, said hangersbeing mounted at spaced positions along said beam assembly andpositioned so that lines of action of tension due to the weight of thesuspended lower riser portions are closely adjacent to, or on, a lineformed by the intersection of said beam assembly at the level of thehangers and the plane defined by said tethers, to minimize or eliminatea turning moment imparted to the beam assembly, which tends to causerotation of the beam assembly around its major axis, as a result of theweight of the suspended lower riser portions.
 24. A mid-water tetheredbuoyant support assembly for a riser system for use in water to bringfluids from seabed equipment to a production vessel at the surface, thetethered buoyant support assembly comprising: at least two tethersextending from seabed anchors, wherein said tethers are located in asingle plane; a beam assembly extending between and connected to thetops of said tethers at connections, such that the connections of saidtethers to said beam assembly are disposed along said beam assembly todefine a line extending between the connections; buoyancy means attachedto said beam assembly for buoyantly supporting said beam assembly, so asto maintain tension in said tethers; and hangers for suspending lowerriser portions, said hangers being mounted at spaced positions alongsaid beam assembly, said hangers being positioned closely adjacent to,or on, a line extending between the connections of said tethers to saidbeam assembly, to minimize or eliminate a turning moment applied to saidbeam assembly, which tends to cause rotation of said beam assemblyaround its major axis, as a result of the weight of the suspended lowerriser portions.
 25. A mid-water tethered buoyant support assembly for ariser system for use in water to bring fluids from seabed equipment to aproduction vessel at the surface, the tethered buoyant support assemblycomprising: at least two tethers extending from seabed anchors, saidtethers defining a plane over at least a portion of their length; a beamassembly extending between and connected to the tops of said tethers;hangers attached to said beam assembly for suspending lower riserportions; and upper riser supports attached to said beam assembly forsuspending upper portions of the riser system, wherein said hangers andsaid upper riser supports are positioned in a radial direction relativeto said beam assembly such that the following condition is met:T·a−t·b≈0 where T equals the tension due to the lower riser portions, aequals the radial distance from the line of action of T to a line formedby the intersection of said beam assembly at the level of the tethersand the plane defined by said tethers, t equals the tension due to theupper portions of the riser system, and b equals the radial distancefrom the line of action of t to the line extending between theconnections of said tethers to said beam assembly.
 26. An assembly asrecited in claim 25, wherein the upper riser portions of the risersystem are flexible.
 27. An assembly as recited in claimed 25, whereinsaid tethers are located in a single plane.